Preliminary studies of macromolecular uptake by the nuclei of intact Xenopus eggs and early embryonic cells have been performed and the following result obtained: Within 5 minutes after microinjecting the cells with 125I-myoglobin or 125I-bovineserum albumin the nuclear concentration of the tracer proteins appeared to be equal to that in the cytoplasm. Ferritin is also able to enter the nuclei of fertilized eggs within 5 minutes after injection. Although not yet definitive these results suggest that, compared to oocytes, there is an increase in nuclear permeability after fertilization and during early development in amphibians. In a second series of experiments, ferritin-conjugated concanavalin A was used as an electron-opaque tracer to study the distribution of oligo-saccharides on nuclear envelopes isolated from amphibian oocytes. Con A was not detected in the pore complexes or along the nucleoplasmic or cytoplasmic surfaces of the envelope. However, lectin binding did occur along the membrane surfaces which line the perinuclear space, and the binding reaction could be inhibited by preincubating the ferritin-conjugated Con A with gamma-methyl-mannoside. BIBLIOGRAPHIC REFERENCE: The localization of the nuclear protein pool in Xenopus oocytes. Feldherr, C.M. and Richmond, P.A. (1976) Exp. Cell Res. 100:417.